In the manufacture of e.g. paper, a fibre web is formed by feeding fibres which are uniformly distributed in water onto or between forming fabrics or by allowing them to be taken up by a fabric-coated cylinder immersed in a tray. The forming fabric consists of a textile fabric of metal or synthetic-fibre yarns. The forming fabric serves two major functions, viz. to separate the fibres from the water and to form the fibres in a manner ensuring that an even and continuous fibre sheet is formed. The interstices between the yarns in the textile fabric form drainage channels through which the water is discharged and consequently these yarn interstices must not be too large since if they are the fibres might be entrained with the water and carried to the so-called white water. The density and surface properties of the fabric are factors which directly determine the quality of the finished paper. Uneven dewatering and uneven fabric surface give rise to irregular fibre formation, and this, in turn, influences the properties of the paper, such as the marking tendencies. Experiments have also been carried out with forming fabrics in the form of perforated plates, but for various reasons these have not found extensive application. The continuous fibre sheet obtained on the forming fabric has a comparatively high moisture content which is reduced by pressing and drying the sheet in the pressing and drying sections. Because of the high energy costs, it is desirable that as great amounts as possible of the moisture are removed in the press section, whereby the heating costs in the drying section can be kept at a minimum. In the pressing operation, the fibre web is compressed between two rollers together with one or several press felts and/or press fabrics. The nature of these is such that the water pressed from the fibre web penetrates into and partly through the felt. The press felt should both protect the fibre web during the pressing operation and lead off the water from the fibre web. The surface structure of the resulting paper is largely dependent on the pressing operation, which in turn is dependent upon the evenness of the press felt. The majority of press felts consists of a base fabric to which is needled a fibre batt. The fibre batt is produced by carding and has in itself a certain degree of unevenness which is amplified by the needled rows which arise in the basic fabric during the needling operation. To produce the best paper quality possible it is necessary that the side of the press felt facing the paper web is as even and finely porous as possible, while at the same time the back should be highly capable of leading-off and removing the water.
Attemps have been made to increase the permeability of the felt and its capacity to absorb moisture by providing in at least one fibre layer a moisture storage in the form of angularly inclined channels. Such channels are produced by melting of the fibre materials. Although this measure may impart improved dewatering properties, the problem nevertheless remains concerning the surface structure of a fibre product. Although at the present time the needled fibre batt gives the best and most even-fibred structure it does not solve the problems caused by streaks formed by the needles or other unevenness in the surface structure that have an effect on the evenness in the pressing operation and result in an undesirable coarseness of the paper surface. Moreover, fibre material structures display irregular, randomly located holes which give the structure or the press felt an uncontrollable porosity which may vary in different parts of the felt. Attempts have been made to grind the surface of fibre structures for the purpose of improving the surface evenness, but this grinding or smoothing operation has given rise to other inconveniences.
Also in the drying section, felts or fabrics are used for the purpose of pressing the fibre web or paper web against heated cylinders. The degree of drying and drying capacity in this section depend upon the evenness of the pressure with which the sheet is pressed against the cylinder and consequently the surface evenness of the felt or fabric is of great importance also in the drying section.
The purpose of the present invention thus is to provide a sheet assembly which may be used as a forming fabric, press fabric and drying fabric, including as a press felt and a drying felt. Prior-art forming fabrics have a surface with knuckles which protrude above the textile structure, bend and again turn downwards. Irrespective of how evenly these knuckles are distributed, it is desirable to produce and use a dewatering device having as even a surface as possible. It is, moreover, desirable that the porosity is as even as possible in order to achieve even dewatering and even formation of the fibre web when the sheet assembly is used as a forming fabric.
Prior-art press felts having a fibre fibrous structure are not very capable of withstanding the dynamic compression which occurs to a great extent in paper making machines in which the press felt is run through several million revolutions while being exposed to heavy loads. This leads to compression of the press felt and an increase of its density. The compression and density of the felt are also caused by weakening of the textile fabric structure, which consists of a large number of intersecting mono- and multifilament threads.
Evenness of the compression pressure also plays a decisive part in the surface structure of the paper as also in the dewatering of the sheet in the press nip. Even if a fibrous structure is ground or smoothed, it will nevertheless display a certain unevenness, which leads to a reduced dewatering effect and to a coarser surface structure in the finished paper. The surface unevenness of the felt or the fabric also increases the possibility for chemical attacks, soiling etc. It is thus desirable to produce a felt or a fabric which possesses as even a surface as possible.
Furthermore, in order to ensure maximum dewatering evenness, it is desirable to provide a high degree of controlled porosity and to be able to predetermine as far as possible the location of the pores. The term "pores" as used herein relates to moisture conductor means. In the majority of fibrous structures incorporating so-called needled felts, it is impossible to avoid that the needles cause agglomeration of fibres upon needle penetration through the batt layer.
In the past, the production of particularly press felts involves a long series of low-production, inexact processes, such as is for example the case in the manufacture of batts. For this reason it is desirable both to reduce the number of processes involved and to improve the accuracy of the processes.